1. Field of the Invention
The present invention relates generally to a receiving detector for detecting coherent radiation, typically a laser beam, while discriminating against noncoherent radiation, and has application in the detection of an installation being illuminated by an unknown laser source, and also in laser communication systems, particularly in improving signal to noise ratios therein. In the first application, the detector is capable of determining the wavelength and angle of incidence of the irradiating unknown source of coherent radiation.
More particularly, the subject invention pertains to a passive coherent radiation detection receiver system in which holographic optical elements are used to discriminate between coherent radiation and noncoherent (bright light) radiation. In greater detail, the radiation detection system is capable of detecting the wavelength and angle of incidence of the laser radiation, and is also capable of attenuating or mitigating the incoming beam intensity level by subdivision of the incident beam into a collection of beams. Pursuant to the latter feature, a detector having an array of pixels such as a CCD camera is not saturated.
2. Discussion of the Prior Art
Advances in high energy laser and optical technology that are being incorporated into some current military scenarios stress the need to address three tactical situations, namely, (1) irradiation by a laser beam from a potential adversary, (2) laser communications in space and (3) laser beam communication systems operating through scattering and attenuating mediums such as water in communications with submerged submarines at sea. For example, a sophisticated naval intelligence collection vessel might utilize a searching, interrogating laser beam directed at a friendly aircraft during flyby. Also, the use of a modulated laser beam for communication through a scattering and attenuating medium such as communication with a submerged submarine is an evolving technology. In this scenario, the problem is not concerned with an encounter with an unknown type of laser beam irradiated by a potential adversary, but instead deals with a communication link via a known type of laser beam with a friendly receiver. The problem to overcome in this case is the severly decreased signal strength due to scattering and attenuation of the light received by the submarine's detection system by the water thereabove. Other applications, such as air-to-air encounters and space-based laser systems, also require distinguishing between coherent laser radiation and other sources of noncoherent radiation, or perhaps one from a set of laser wavelengths. These applications require an ability to efficiently collect, store, concentrate and analyze laser signals as they are encountered.
Several technical problems are common to tactical situations arising as a result of the evolving use of lasers for interrogation, intelligence collection, and data transmission, and include discrimination of laser radiation, low signal to noise levels, a range of distinct operating wavelengths, directional sensitivity to source location, propagating beam spatial degradation, accommodating continuous wave or pulsed operation, and real time response.
The present invention provides an ability to detect laser irradiation in the presence of competing, noncoherent background radiation, to measure its wavelength and to determine the angular direction of the irradiating laser beam. At times, the overall background level of noncoherent radiation, which is essentially noise, could be comparable to or even greater in strength than the laser beam, thereby complicating the detection process.